This invention pertains to the field of ventilatory support for respiratory failure, particularly due to lung disease, and in particular to automatically providing sufficient end expiratory pressure to unload intrinsic positive end expiratory pressure (PEEPi)
Subjects with chronic airflow limitation (CAL) due, for example, to emphysema and chronic bronchitis, may require ventilatory assistance, particularly during periods of acute exacerbation, or routinely at night.
Ventilatory support can reduce the work of breathing, reduce the sensation of breathlessness, and improve blood gases (oxygen and carbon dioxide levels). In subjects with CAL, most of the work of breathing is due to the high airway resistance. Approximately two thirds of this resistance is relatively fixed, and due to narrowing of the airways. However, of the order of one third of the resistance is due to dynamic airway compression during expiration. Dynamic airway compression occurs when the pleural pressure exceeds the pressure in the lumen of the airway during expiration, causing flow to become independent of effort.
In a normal subject, the alveolar pressure decays exponentially during expiration, so that the expiratory flow and alveolar pressure (relative to atmospheric) are both approximately zero at the end of expiration, and the lungs and chest wall have returned to their passive equilibrium volume VR. In patients with CAL, however, as a result of dynamic airway compression and fixed reduced expiratory flow rate, it is not possible for the lungs to return to VR in the time allowed before the start of the next inspiration. The chest is hyperinflated. The alveolar pressure remains positive, on the order of 5 to 15 cmH2O at the end of expiration. This raised alveolar pressure is termed intrinsic positive end expiratory pressure, or PEEPi. (Other names for the phenomenon are covert PEEP and occult PEEP.)
An important effect of the hyperinflation is that the patient must overcome the elastic recoil of the hyperinflated chest wall before inspiratory airflow can commence. The PEEPi is said to act as an inspiratory threshold load. A further undesirable effect of PEEPi is that during artificial mechanical ventilatory support, it interferes substantially with the triggering of the ventilator, causing patient-machine asynchrony.
It is now well understood that the addition of a counterbalancing external positive end expiratory pressure (called external PEEP, or just PEEP), approximately equal in magnitude to PEEPi, is of great benefit. First, it prevents dynamic airway compression, permitting greater expiratory airflow. Second, it balances the inspiratory threshold load. Third, it improves triggering of a ventilator by the patient.
Use of excessive PEEP, however, can be disadvantageous and even dangerous. Excessive PEEP above and beyond PEEPi will cause yet further hyperinflation. This will result in stiffening of the lung and chest wall, and an increase in the elastic work of breathing. It will also cause reduced cardiac output, and can lead to barotrauma. Further, the peak inspiratory airway pressure during ventilatory support cannot be arbitrarily increased without either exceeding the capacity of the ventilator, or reaching a pressure that is itself dangerous. Finally, excessive external PEEP will also reduce the possible airway pressure excursion or headroom available for lung inflation.
Therefore, it is advisable when applying external PEEP to set the external PEEP as close as possible to PEEPi. Since PEEPi varies from time to time, depending on a number of factors including, for example, the resistance of the small airways and the respiratory rate, both of which change with changing sleep stage, chest infection, or bronchospasm, it is desirable to be able to make multiple, or even continuous, measurements of PEEPi in order to optimize external PEEP.
A typical patient in an intensive care unit is heavily sedated and paralyzed during ventilatory support, and it is straightforward to measure the PEEPi. It is necessary only to occlude the airway during late expiration, and measure the airway pressure, which, after a few seconds of equilibration, will equal static PEEPi. Since the lung injury in CAL is usually markedly heterogeneous, different alveoli will have different end expiratory pressures, and static PEEPi is therefore a weighted average across all alveoli.
Another known method which is suitable for use in the paralyzed sedated patient is to measure the airway pressure at the start of machine inspiratory effort, and again at the start of actual inspiratory airflow. The difference between these two pressures is the dynamic PEEPi. Dynamic PEEPi reflects the end expiratory pressure in the least abnormal lung units, and substantially underestimates static PEEPi.
These simple methods do not work for patients who are not sedated and paralyzed, and who are making spontaneous breathing efforts, because they do not take into account the patients"" own respiratory muscle efforts.
One known method that is used with such patients requires a Muller manoeuvre (maximal inspiratory effort) during catheterization of the oesophagus and stomach, and is therefore completely unsatisfactory for repeated or continuous measurements in the ambulatory patient or the patient who is being treated at home long-term.
Methods for measuring the airway conductance in spontaneously breathing patients using oscillometry are taught by Peslin et al., Respiratory Mechanics Studied by Forced Oscillations During Mechanical Ventilation, Eur Respir J 1993; 6:772-784, and by Farre et al., Servo Controlled Generator to Measure Respiratory Impedance from0.25 to 26Hz in Ventilated Patients at Different PEEP Levels, Eur Respir J 1995; 8:1222-1227. These references contemplate separate measurements for inspiration and expiration. Oscillometry requires modulation of the airway pressure at a high frequency, such as 4 Hz, and measurement of the resultant modulation of the respiratory airflow at that frequency. However, these references fail to describe servo-controlling of ventilation to increase or decrease PEEP so that the inspiratory arid expiratory conductances are approximately equal.
Oscillometry has been used to control nasal CPAP (see U.S. Pat. No. 5,617,846) or bilevel CPAP for the treatment of obstructive sleep apnea (see U.S. Pat. No. 5,458,137). The problem there is essentially opposite to the problem under consideration here. In obstructive sleep apnea, there is increased resistance during inspiration, and the above two patents teach that increased resistance during inspiration can be treated by an increase in pressure. In patients with CAL and dynamic airway compression, there is increased resistance during expiration.
There is no known method or apparatus which can automatically or continuously control a ventilator or CPAP apparatus in conscious spontaneously breathing patients in order to prevent expiratory airflow limitation or to unload PEEPi in CAL.
Yet another known method for estimating PEEPi, taught, for example, by Rossi et al., The Role of PEEP in Patients with Chronic Obstructive Pulmonary Disease during Assisted Ventilation, Eur Respir J 1990; 3:818-822, is to examine the shape of the expiratory flow-volume curve, which has been observed to be exponential, if there is no dynamic airway compression. The reference further notes that in the absence of PEEPi, the flow-volume curve becomes a straight line.
The above known art only contemplates the application of an external pressure which is constant during any one expiratory cycle. However, the elastic recoil of the lung is higher at high lung volume, and lower at low lung volume. Therefore, it may be advantageous to find the minimum external pressure at each moment in time during an expiration that will prevent dynamic airway compression during that expiration.
It is an object of our invention to vary the ventilatory pressure during expiration as a function of the degree of the patient""s dynamic airway compression.
It is another object of our invention to vary the ventilatory pressure automatically based solely on continuous measurements that are already taken in conventional CPAP and ventilator apparatuses.
The present invention seeks to provide continuous and automatic adjustment of the expiratory pressure during ventilatory support, so as to substantially prevent dynamic airway compression and unload intrinsic PEEP with the smallest amount of external expiratory pressure.
The basic method of the invention prevents dynamic airway compression during ventilatory support using a conventional interface to a patient""s airway such as a face mask, nose mask, or endotracheal or tracheotomy tube, and providing the interface with an exhaust and a supply of breathable gas at a variable pressure as is known in the CPAP and ventilatory arts. The respiratory airflow is determined by measurement or calculation, and a measure of the degree of dynamic airway compression is derived. This measure is servo-controlled, preferably to be zero, by increasing expiratory pressure if the measure of the degree of dynamic airway compression is large or increasing, and by reducing expiratory pressure if the measure of the degree of dynamic airway compression is small or zero.
The measure of the degree of dynamic airway compression may be an instantaneous or pointwise measure within any given breath, and the step of servo-controlling the measure to be zero may similarly be performed pointwise within a given breath, so that the expiratory pressure is similarly varied pointwise within a breath. As an alternative to thus basing the airway compression determination and the servo control on multiple airflow determinations made within each individual respiratory cycle, the derivation of the measure of the degree of dynamic airway compression and the servo-controlling of the airway compression may be performed across a plurality of respiratory cycles.
During expiration, the expiratory pressure increase may be linear as a function of expired volume as will be described below.
The measure of the degree of dynamic airway compression is preferably derived by measuring the airway conductance separately during the inspiratory and expiratory portions of one or more respiratory cycles, and calculating the measure of the degree of dynamic airway conductance as a function of the inspiratory conductance minus the expiratory conductance, or alternatively as the ratio of the inspiratory conductance to the expiratory conductance. The two separate conductances during inspiration and expiration may be measured by superimposing a high-frequency oscillation on the patient interface pressure, at a known or measured amplitude, identifying the inspiratory and expiratory portions of each respiratory cycle, measuring the component of the respiratory airflow at the high frequency separately over the inspiratory and expiratory portions of one or more respiratory cycles, and from these measurements and the determined pressure amplitude calculating the inspiratory airway conductance and the expiratory airway conductance.
Alternatively, the measure of the degree of dynamic compression may be derived from the shape of the expiratory airflow versus time curve. The measure is zero when the expiratory flow decays exponentially from the moment of the peak expiratory flow to end expiration, but is large when the expiratory flow decreases suddenly from the peak expiratory flow and is then steady but non-zero for the remainder of expiration. The measure may be the ratio of the mean expiratory flow during approximately the last 25% of expiratory time to the peak expiratory flow.